The sensor art includes a wide variety of devices designed to measure the physical position of mechanical elements, and to translate the position information into an electrical signal. Examples include the LVDT (linear variable differential transformer or transducer), the RVDT (rotary variable differential transformer), resolvers, selsyn (self-synchronous) devices, resistive and inductive potentiometers, variometers, and a variety of variable reluctance sensors.
In many applications, there are advantages to be gained from replacing traditional electrical sensors with optical sensors that are totally nonelectrical and that use fiber-optic cables rather than wire for their interconnections. An important advantage of such sensors is that they are immune to the threat of natural and man-made electrical interference. This feature is particularly desirable on aircraft where heavy electrical shielding is normally required to protect the wire interconnections of sensitive electrical sensors from the effects of lightning. A second important advantage is that optical sensors, when fully developed, will be lighter, less costly, and more reliable than their electrical counterparts.
FIG. 1 illustrates an example of an analog optical rotary position sensor, that operates by intensity-modulating a light beam. In this sensor, light radiating from fiber-optic cable 12 is collimated by lens 14, and the collimated beam passes through encoder 16 that includes substrate 18 on which track 20 has been deposited. The encoder is mounted to shaft 22 whose rotational position is to be determined. Track 20 has an optical density that is a function of rotary position around the encoder. The particular function relating position to density may be linear, or have any other prescribed form.
Light passing through the encoder 16 is collected by lens 24 and focused into fiber-optic cable 26. In this example, the light arriving at fiber-optic cable 26 varies in intensity as a function of rotation of the encoder and shaft. The same principle may be applied equally well to a linear position sensor in which the encoder is guided in a straight-line path past the optical system. Similar principles may also be applied to sensors of temperature, pressure, and other physical parameters, if those sensors are made to employ optically readable moving encoders.
FIG. 2 partially illustrates a conventional digital optical rotary position sensor. The sensor includes encoder 30 that comprises substrate 32 having a series of parallel, side-by-side tracks 34C-34G. A corresponding number of fiber-optic cables 36 are positioned with their ends adjacent to the respective tracks, for illuminating the tracks. In a reflective embodiment, the fiber-optic cables also receive the light reflected from the tracks. The tracks consist of alternating reflecting and nonreflecting segments, typically arranged in a Gray code pattern. Encoder 30 moves parallel to the tracks, such that each encoder position produces a unique set of binary digits to the detection system.
As described above, the encoder generally consists of a substrate material upon which a code pattern has been formed. The code pattern may be either analog (as in FIG. 1) or digital (as in FIG. 2), and may be designed to be read optically either by transmitted light or by reflected light. The performance of all optical sensors of this type, whether analog or digital, is sensitive to imperfections in the encoder itself, and in the optical components that are used to read or interpret the code pattern. In an analog sensor, imperfections degrade the measurement accuracy or resolution. In a digital sensor, imperfections frequently cause nonuniform responses from the various tracks, thereby creating a dynamic range problem which increases potential measurement error in the associated electronics.
Typically, the first step in the production of an encoder is the production of artwork for the code pattern, generated either manually or by using computer-aided design techniques. The code pattern is then transferred to a photomask by means of photolithographic techniques. If the pattern is generated oversize to achieve the required accuracy or resolution, it is first photoreduced to the proper size. The substrate is vacuum coated with a contrasting film, usually metallic, and the coated substrate is overcoated with a photoresist, which is then dried and cured. The photomask is then aligned with the substrate, and the photoresist is exposed to light through the photomask. The exposed substrate is then separated from the photomask, and is developed and washed, leaving clear areas where the contrasting coating is to be removed. The contrasting film is removed by chemical etching through the clear areas of the photoresist, and the remaining resist is then removed, leaving the desired contrasting code pattern upon the substrate.
Encoders may also be produced by a "lift-off" process in which resist is first applied to the substrate, and then exposed and developed. The contrasting film is then deposited over the resist. The resist is then lifted off, leaving contrasting film on the substrate in the clear regions of the resist.
The above procedures are complex, and unacceptable pattern imperfections may develop during any one or more of the many steps involved. In addition, the optical system needed to read the pattern will typically contain a number of optical elements (e.g., lenses, gratings, prisms, etc.), each of which can have imperfections that may also degrade the accuracy of the total system. As a result, it has in the past proved quite difficult to produce precision optical position sensors with a high manufacturing yield.